The present invention relates to active target distance measurement and more particularly but not exclusively to active distance measurement to a target using reflection of electromagnetic pulses.
Distance measurement to a target object is necessary for a wide range of applications. Methods for distance measurements include both active and passive methods. Passive methods are generally based on the observation of the object to be measured from at least two points and then calculation based on the observations using triangulation techniques. Active methods generally involve reflecting a beam from the object and measuring the time delay in the round trip. The distance may then be calculated using the identity
speed=distance/time.
In this case,
distance=beam speedxc3x97measured delay/2.
The measured delay is referred to hereinbelow as Time of Fly.
Other methods measure the phase of the transmitted pulse in order to determine the distance.
The time of fly method requires extremely high measurement accuracy and thus relatively complex electronics. It is thus impractical to produce time of fly based apparatus as an array to obtain spatial information from a plurality of points on an object. For applications such as robotics and robot navigation, and for multimedia applications, distance to a given point needs to be known from many points of a three dimensional object.
Currently, obtaining three dimensional location information of an object is possible using two cameras whose intervening distance is known in three dimensions, and then carrying out triangulation, generally separately for each dimension. However, such a method requires considerable calculation and is difficult to carry out in real time.
The present embodiments overcome the problem of the prior art by measuring the energy of the reflected beam over a gating interval to produce a gated energy. The gated energy is compared with a calibration energy to produce a ratio which is inversely proportional to the object distance.
Energy of a beam is the integral of the brightness or amplitude of the beam and is a measurement which can easily be obtained from a detector such as a charge coupled device (CCD).
According to a first aspect of the present invention there is provided a method of measuring distance to a target comprising
reflecting a pulse off a target,
retrieving energy from the reflected pulse over a predetermined gating period and measuring a level of the retrieved energy,
determining a distance of the target by comparing the measured retrieved energy level from the target over the predetermined gating period with a calibration energy level.
Preferably, the method comprises carrying out the step of retrieving at each one of a plurality of sensors.
Preferably, the step of retrieving energy comprises detecting a brightness of the reflection and integrating the brightness over the predetermined gating period.
Preferably, the predetermined gating period is a length of the pulse.
Preferably, the step of determining comprises forming a ratio between the calibration energy level and the gated energy, which ratio is inversely proportional to the distance.
Preferably, the duration of the pulse is set at a time taken by light to travel the maximum distance it is intended to measure.
Preferably, the calibration is carried out by measuring a total energy of a pulse reflected from the object.
Preferably, the calibration and the gated energy retrieval are carried out on a single reflected pulse.
Preferably, the method comprises a step of splitting the reflected pulse into two, one part being used for calibration energy retrieval and one part being gated for gated energy retrieval.
Preferably, two successive pulses are reflected one after the other, a first beam being used for calibration energy retrieval and the second being gated for gated energy retrieval.
Preferably, the method comprises splitting incoming light into two parts, one part for analysis of the reflected pulse to determine distance information and a second part being used to obtain image information.
Preferably, the image information is color information.
Preferably, the pulse is any one of a group comprising a light beam, an ultra-violet beam, an infra-red beam, a microwave beam and an ultra-sound beam.
According to a second aspect of the present invention, there is provided apparatus for determining a distance of a target, the apparatus comprising a pulse transmitter, a gatable beam detector, and a comparator, the gatable beam detector being operable to obtain gated and calibration beam energy information of a pulse transmitted by the beam pulse transmitter for reflection from the target, and to pass the beam energy information to the comparator, thereby to obtain a ratio between the gated and the calibration beam energy, the ratio being inversely proportional to the distance.
Preferably, the pulse beam transmitter is operable to transmit a pulse having a predetermined width and the gatable beam detector is operable to be gated at at least twice the predetermined width and at the predetermined width, thereby to produce a total reflected energy of the pulse for the calibration energy, and the gated pulse energy.
Preferably, the detector comprises two sub-detectors, one gated at the at least twice the predetermined width and the other gated at the predetermined width.
The apparatus preferably comprises a beam splitter for splitting the reflected pulse between the first and the second sub-detectors.
The apparatus preferably comprises a beam splitter for splitting light incoming to the apparatus between a first sub-detector gated to obtain distance information and a second sub-detector for obtaining general image information.
The apparatus preferably comprises compensation factors to correct for built-in errors.
Preferably, the detector is part of an array of detectors.
Preferably, the array is a three-dimensional array.
According to a third aspect of the present invention there is provided an array of distance sensors for sensing distance to multiple points of a target object by detecting pulses reflected off a target object, each sensor comprising a gatable detector for detecting incident energy levels of said pulses to produce an output indicative of the distance of a respective point.
Preferably, the sensor is gatable to a time delay equaling at least twice of a length of the pulse to obtain a calibration energy, and the sensor is further gatable to a time delay equaling a length of the pulse to obtain an energy level indicative of the distance.